1. Field of the Invention
This invention relates to radio receiving and transmitting systems which adaptively suppress interference by controlling the null directivity of the pattern of an antenna array.
2. Description of the Prior Art
Adaptive array antenna systems are currently the subject of intense interest and investigation/development for radar and communications applications. The principal reason for the interest is their ability to automatically steer nulls onto undesired sources of interference, thereby reducing output noise and enhancing the detection of desired signals. These systems usually consist of an array of antenna elements and a real-time adaptive receiver-processor which has feedback control over the element weights.
Communications and radar antenna systems using either conventional narrow band or spread spectrum modulation formats are susceptible to degradation in SNR performance caused by undesired "noise" which intrudes via the antenna sidelobes and mainlobes. The noise may consist of deliberate electronic countermeasures (ECM) friendly RF interference (RFI), clutter scatterer returns, and natural noise sources. This degradation is often further aggrevated by motion of the antenna, poor sitting conditions, multipath, and a changing interference environment. Adaptive array techniques offer possible solutions to these serious interference problems via their flexible capabilities for automatic null steering and notching in the spatial domain, the frequency domain, and in polarization. Adaptive nulling is considered to be the principal benefit of adaptive techniques at the present time.
A tutorial paper on "Adaptive Arrays--An Introduction" by William F. Gabriel appears in the Proceedings of the IEEE Vol. 64, No. 2, February 1976, pages 239-272. United States patents of interest includes: U.S. Pat. No. 3,713,167--David--Class 343/797 U.S. Pat. No. 3,932,818--Masak--Class 328/167 U.S. Pat. No. 3,981,014--Masak--Class 343/100LF U.S. Pat. No. 4,024,541--Albanese et al.--Class 343/17.1R U.S. Pat. No. 4,064,422--Masak--Class 364/841 U.S. Pat. No. 4,097,866--Frost et al.--Class 343/100LE U.S. Pat. No. 4,075,633--Lewis--Class 343/100LF U.S. Pat. No. 4,015,977--Fitting et al.--Class 325/472
It is also possible that an unfriendly source of deliberate interference may possess the capability for monitoring the transmissions from the transceiver. It is therefore desirable not only to protect the receiver from the interference, but to also prevent information from the transmitter from being intercepted.
The least mean square (LMS) algorithm has been firmly established for adaptive antenna systems. A hard-limiter modification has been introduced into the basic control loop to improve the circuit characteristics, in particular the system dynamic range is doubled.